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Identity-crisis comet may really be closest asteroid to the sun

18 May 2016

By Jacob Aron

WHEN is a comet not a comet? It’s a question astronomers are asking themselves more and more often. Now it seems one of these supposed ice balls might actually be an asteroid that gets within a cosmic hair’s breadth of the sun – a mere 8 million kilometres from it.

The two kinds of space rocks are traditionally thought to be very different. Comets are loose piles of rock and ice on long, elliptical orbits that heat up and develop a tail of gases as they near the sun. Asteroids, on the other hand, are lumpy bodies of hard rock and metal that mostly orbit the sun at a distance that falls somewhere between Mars and Jupiter.

But an increasing number of objects are being discovered that blur the line between the two. The latest is comet 322P/SOHO 1, discovered in 1999 by NASA’s Solar and Heliospheric Observatory. But SOHO’s view is shaded to protect it from intense sunlight and its resolution is comparatively low, meaning it can’t get a good look at 322P during the comet’s closest approach to the sun.

“A rising number of objects are being discovered that blur the line between comets and asteroids“

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Now, Matthew Knight at the University of Maryland in College Park and his colleagues have used ground-based telescopes and the Spitzer space telescope to take another look. They found there was no sign of a tail from 322P as it got close to the sun. They also found that its density is at least 1000 kilograms per cubic metre, double that of the famous comet 67P/Churyumov-Gerasimenko (The Astrophysical Journal Letters, doi.org/bhjj).

The readings are a big clue that 322P may actually be an asteroid, says Knight. If so, that would make it the asteroid that gets closest to the sun, coming to within about 5 per cent of the distance between Earth and the sun.

Knowing where the line falls between asteroid and comet is useful in helping us trace the history of the solar system.

This article appeared in print under the headline “Sun-skimming comet might be an asteroid”